1. Field
The present invention relates to a radial tire intended to be fitted to a heavy vehicle of construction plant type.
2. Description of Related Art
Although not restricted to this type of application, the invention will be more particularly described with reference to a radial tire intended to be mounted on a damper, a vehicle for transporting materials extracted from quarries or open cast mines. The nominal diameter of the rim of such a tire, within the meaning of the European Tire and Rim Technical Organisation (ETRTO) standard, is a minimum of 25″.
The following definitions apply in what follows:                “Meridian plane”: a plane containing the axis of rotation of the tire.        “Equatorial plane”: the plane passing through the middle of the tire tread surface and perpendicular to the axis of rotation of the tire.        “Radial direction”: a direction perpendicular to the axis of rotation of the tire.        “Axial direction”: a direction parallel to the axis of rotation of the tire.        “Circumferential direction”: a direction perpendicular to a meridian plane.        “Radial distance”: a distance measured perpendicular to the axis of rotation of the tire and from the axis of rotation of the tire.        “Axial distance”: a distance measured parallel to the axis of rotation of the tire and from the equatorial plane.        “Radially”: in a radial direction.        “Axially”: in an axial direction.        “Radially on the inside, or respectively radially on the outside”: which is situated at a smaller or greater radial distance, respectively.        “Axially on the inside, or respectively axially on the outside”: which is situated at a smaller or greater axial distance, respectively.        
A tire comprises two beads which provide the mechanical connection between the tire and the rim on which it is mounted, the beads being joined respectively by two sidewalls to a tread intended to come into contact with the ground via a tread surface.
A radial tire more particularly comprises a reinforcement, comprising a crown reinforcement radially on the inside of the tread, and a carcass reinforcement radially on the inside of the crown reinforcement.
The carcass reinforcement of a radial tire for a heavy vehicle of construction plant type usually comprises at least one carcass reinforcement layer made up of metallic reinforcing elements coated in a polymer coating material. The metallic reinforcing elements are substantially parallel to one another and make an angle of between 85° and 95° with the circumferential direction. The carcass reinforcement layer comprises a main part, joining the two beads together and wrapped, within each bead, around a bead wire. The bead wire comprises a circumferential reinforcing element, usually made of metal, surrounded by at least one material, which, and this list is not exhaustive, may be made of polymer or textile. The carcass reinforcement layer is wrapped around the bead wire from the inside towards the outside of the tire to form a turn-up having an end. The turn-up, within each bead, allows the carcass reinforcement layer to be anchored to the bead wire of that bead.
The end of the turn-up is often covered, on both its axially inner and axially orate faces respectively, by an edging element made of a polymer edging material, usually of the same chemical composition as the polymer coating material, but which may be different. The edging element thus constitutes an additional thickness of polymer coating material at the end of the turn-up.
Each bead also comprises a filling element which extends the bead wire radially outwards. The filling element is made of at least one polymer filling material. The filling element may be made of a radial stack of at least two polymer filling materials which are in contact along a contact surface that intersects any meridian plane along a meridian line. The filling element axially separates the main part from the turn-up.
A polymer material, after curing, is mechanically characterized by tensile stress-deformation characteristics which are determined by tensile testing. This tensile testing is carried out by a person skilled in the art, on a test specimen, according to a known method, for example in accordance with international standard ISO 37, and under standard temperature (23° C.+ or −2° C.) and moisture (50%+ or −5% relative humidity) conditions defined by international standard ISO 471. For a polymer material, the elastic modulus at 10% elongation, expressed in mega pascals (MPa), is the name given to the tensile stress measured for a 10% elongation of the test specimen.
A polymer material, after curing, is also mechanically characterized by its hardness. The hardness is notably defined by the Shore A hardness determined in accordance with standard ASTM D 2240-86.
When the vehicle is being driven along, the tire, mounted on its rim, inflated and compressed under the load of the vehicle, is subjected to bending cycles, particularly in its beads and its sidewalls.
The bending cycles lead to variations in curvature which are combined with variations in the tension of the metallic reinforcing elements of the main part and of the turn-up.
Considering that a bead behaves mechanically in bending like a beam the respectively outer and inner axes of which are the main part and the turn-up, the turn-up, subjected to the bending cycles, undergoes compressive deformations likely to lead to its fatigue failure and therefore to a reduction in the endurance of the bead and in the life of the tire.
Document EP 2 216 189 describes a tire bead the endurance of which is improved by reducing the compressive deformations in the turn-up when the bead flexes on the rim when in use. This objective is achieved by a turn-up which is such that the distance between the turn-up and the main part decreases continuously, radially towards the outside, from the bead wire, as far as a minimum distance, and then increases continuously as far as a maximum distance. The turn-up extends radially on the outside of that point of the turn-up that corresponds to the maximum distance between the turn-up and the main part.
The bending cycles also lead to stresses and deformations in the polymer coating, edging and filling materials situated in close proximity to the end of the turn-up.
More specifically, the stresses and deformations in close proximity to the end of the turn-up cause cracks, which start at the end of the turn-up, to spread, more particularly in the case of reinforcing elements made of metal, these cracks being likely to lead to a reduction in the endurance of the bead and in the life of the tire.
According to the inventors, the cracks start mainly as a result of a lack of adhesion between the ends of the metallic reinforcing elements of the turn-up and the polymer coating, edging or filling materials in contact with the said ends. The increase in bead temperature, during the bending cycles, accentuates the lack of adhesion that already exists in the new tire.
The cracks spread through the polymer coating, edging and filling materials and lead to degradation of the bead, and therefore failure of the tire. The rate at which the cracks spread depends firstly on the amplitude and frequency of the stress and deformation cycles, and secondly on the respective stiffnesses of the to polymer coating, edging and filling materials in the crack zone.
Document U.S. Pat. No. 3,921,693 has already described, in the case of a tire with a radial carcass reinforcement the reinforcing elements of which are metallic, beads the design of which is aimed at preventing cracks at the ends of the turn-up. In the technical solution proposed, the end of the turn-up is covered with a polymer material the Shore A hardness of which is greater than that of the polymer filling material or materials.
Document U.S. Pat. No. 4,086,948 has also described, with a view to increasing the life of a radial tire for a heavy vehicle, a turn-up that is tall, i.e. the end of which is radially on the outside of the straight line passing through the axially outermost points of the sidewalls of the tire. In addition, the polymer coating material of the metallic reinforcing elements of the carcass reinforcement has a Shore A hardness and a modulus of elasticity at 300% elongation that are respectively higher than the Shore A hardness and than the elastic modulus at 300% elongation of the polymer filling material.
Finally, document U.S. Pat. No. 5,056,575 describes a bead of a tire for a heavy vehicle, such as trucks and buses, that makes it possible to reduce deformations and slow the spread of cracks in the polymer material in the vicinity of the end of the turn-up, with a view to increasing the endurance of the bead. The technical solution proposed is a bead that has three polymer filling materials of which the elastic moduluses at 100% elongation decrease from the polymer filling material adjacent to the turn-up which is also the radially outermost, to the polymer filling material adjacent to the bead wire which is also the radially innermost.
The inventors have set themselves the objective of increasing the endurance of the beads of a radial tire for a heavy vehicle of construction plant type by reducing the cracking that starts at the end of the turn-up and spreads through the polymer coating, edging and filling materials.